Formally, a vassal is a person who has entered into a mutual obligation to a liege or monarch in the context of the feudal system that represents the core of Crusader Kings. The obligations often included military support and mutual protection, in exchange for certain privileges, usually including the grant of land held as a fiefdom. The term can be applied to similar arrangements in other feudal societies. In contrast, a fidelity, or fidelitas, was a sworn loyalty, subject to the king.
What are Vassals?[]
Your vassals are the nobles who help you run your lands (in exchange for being able to establish a power base within those lands) and allow you to administrate the realm and raise armies and tax from that realm without everything going to pieces. You need vassals because of your demesne limit, or more prosaically, because your liege can't be everywhere at once and things would fall into chaos.
Realm Tree of the Holy Roman Empire (Vassal-Liege feudal relationships).
Vassals, then aren't something you can get rid of. And as most of your army will come from what your vassals volunteer you, hacking them off is one of the fastest ways for a king (or duke) to lose his crown, much like in the original game. Thankfully, your vassals generally won't come hunting for you militarily unless you do something to genuinely aggrieve them. They do, after all, have a stake in the status quo, and this isn't like the first Crusader Kings where it was more a question of when, not if, you got into a fight with the stewards of your kingdom. This will not, however stop some of them; be particularly wary of ambitious vassals, ones who have a personal grudge against you, or younger sons who resent their older brother getting all the kings stuff once he leaves for the great door in the sky (or more likely, the blazing inferno below.)
More likely is that a moderately unpopular king will start to get a swathe of plots against him. This can range from trying to kill you off, to trying to reduce your authority, right up to trying to take your throne off you. Keep a good spymaster on hand so you don't get surprised by these; most vassals will break off the plot if asked nicely, and you get a favourable chance at throwing them in jail if they won't acquiesce to your demands. Its hard to plot efficiently from jail!
Types of Vassal[]
You get two major types of vassal; Counts, the junior nobility who generally command single demesnes or maybe a (very) small number of them, and Dukes, who, in addition to their own demesnes, command loyalty from the counts who hold land within the bounds of their Duchy, and who answer only to the king. This chain of command goes from the minor vassals, the mayors, barons and bishops who command the cities, baronies and bishoprics within your counties, who are in turn commanded by the count or duke in charge of the county, with counts answering to a duke or, if there is no duke, answering directly to the king, and finally the king is who the duke takes orders from. Kings may also be under the command of an emperor.
A word on minor vassals. These are the folks who command the holdings within your county. Mayors command cities, barons command baronies and the bishops command bishoprics. Mayors are odd in that they are elected and will be replaced periodically. You can reassign these vassals to the wrong kind of holding... but they'll take a penalty for it.
Its worth checking these guys periodically if you're at war. Sometimes the best commander you have may be the insignificant mayor of the little village down the coast.
Vassal Opinion[]
Vassal opinions come in two flavours; stuff you can do something about and stuff you can't. Sadly, the stuff in the latter category usually outweighs the stuff you can do something about.
The Stuff You Can Do Something About.[]
Rule 1: Don't get excommunicated, murder someone, execute those poor wretches in your jail (even if they killed your adorable two year old puppy... or son...) or generally run around channelling Caligula. This is where the really nasty modifiers come from; excommunication will earn you -30, and being a known murderer or an executer of prisoners will give you penalties in the 20's. Act like a monster and you'll be treated as one.
Rule 2: Don't demand too much of your vassals. If you want to keep them on side, keep crown authority low, don't tax them too much, and don't take every soldier they have. Yes, this puts restrictions on your power, but an easily ignorable king is one who is unlikely to draw unwanted attention. Of course, keep your demands from your vassals too lenient and you may find yourself short an army and money to pay it with! A balance, as always, is essential. Finally, don't keep them at war all the time; vassals will steadily get angered at the stream of money falling out their strongbox into the pockets of the soldiers around them, money the king never seems eager to reimburse them for!
Rule 3: Finally, try to avoid hacking them off personally with events. If you make a promise, keep it, don't start up an affair with their wives, avoid humiliating them when they make a fool of themselves in court (even if their detractors are right!)... basically, when deciding on a course of action during events, be careful to consider exactly who you are making a monkey out of in front of the entire royal court. If it comes down to a choice between two vassals, annoy the one you can most afford to be displeased with you... or possibly the one who will forgive you due to already worshipping the ground you walk on.
The Stuff You Can't...[]
You can't really help if your king is a royal brat. Having a new ruler will get under the skin of your nobles as he or she has not earned their respect as ruler yet. Having certain traits (being slothful or craven, for example, gives a -10 penalty from every vassal, though there are more) will annoy everyone; there are certain things that people just don't accept about a king. A further -10 will come from having opposing traits to some of your vassals; this can even extend to traits which are otherwise positive to have. Being a foreigner will also detract from your status in the eyes of your peers; watch out if you inherited or "inherited" someone elses crown.
Vassals in Wartime[]
Smart leaders will note that vassals are a ready source of soldiers, and specifically, free soldiers. While this may sound like a sound strategy for cynical kings (as well as a classic method of world conquest by proxy in the original Crusader Kings) your vassals will get sick of being out in the field and will slowly come to resent you for it. This starts off as a fairly mild warning of a few easily reversed points of lost opinion, but don't push it.
A Word of Warning for Would-be Warmongers.[]
Its not easy to discern from the game mechanics, but one thing of note is that vassals tend to get a lot more hostile if you drag them off into wars. Especially if they're long, violent, and aggressive wars fought on the Spanish peninsula (conquerors heaven) over lands of dubious relevance to vassals sitting safe at home miles away from the Christian-Muslim front line. Quite why this happens is not clear, besides your followers annoyance at long periods of military service, but it could have something to do with your armies being a long way away from home, caught in protracted sieges where a lot of prestige is at stake, and probably weakened by attrition and warfare; kings with weakened levies which are far away from home might just fall to a well-timed strike; even a mighty conqueror can fall if his armies are miles away while those he thought were friends storm the once mighty capital.